The output of a laser diode is an asymmetric, highly divergent cone of light which in cross section resembles an ellipse. Lateral spread of the divergent light is generally 25-35 degrees by 8-10 degrees for the major and minor axes of the ellipse. Most laser applications require a collimated symmetric beam of light, and therefore the natural divergence of the laser diode must be corrected. Energy losses can be large if the elliptical beam is not redirected and focused.
Currently, the divergent light spread is predominantly redirected through the use of refracting optics. Compound lenses, usually triplets, and more elaborate systems of collimator lenses and anamorphic prism pairs are used. For example, Seastar Optics (Sidney, British Columbia, Canada) provides a coupling device which includes a pair of diffraction-limited glass aspheric molded lenses. The smaller of the two lenses, a high-numerical-aperture collimator, collects nearly all of the available laser diode light. Then the larger lens directs the light into the fiber. Coupling efficiencies of greater than 50 percent for single-mode applications are thus obtained (G. Rogers and M. Fitch, Photonics Spectra, 175-176, September 1991). In order to compensate for light losses, due to the coupling efficiency being less than 100 percent, higher milliwatt laser diodes are used. These higher milliwatt laser diodes are more powerful and therefore input more light into the system. More light is therefore input into the fiber, even though the coupling efficiency has not been increased. The need for higher milliwatt laser diodes contributes both bulk and temperature problems to the coupling system.
Melles Griot Co. offers a system which utilizes lenses combined with a series of mounted beam expanders, collimators and a mounted anamorphic prism pair to correct the asymmetric output of the laser diode. Once again this lens system is bulky and costly.
Further problems associated with the redirection of laser diode emissions using lenses involve spherical aberration and back reflection of light to the laser diode. Lenses typically reflect back about 4% of the light, degrading the intensity of the laser diode beam. Lenses are also devised for use at optimum wavelengths, and the wavelength of the laser diode emission can be altered by temperature changes and by the back reflection. Once the wavelength of the laser diode shifts, the lens system may no longer be appropriate for the new wavelength. Lens systems also tend to lose part of the elliptical beam when focusing to a circular collection site.
In an attempt to overcome these problems, U.S. Pat. No. 4,981,343, issued Jan. 1, 1991, to Folsom and entitled "Focusing Mirror Lens", provides a monolithic, transparent, preferably cylindrical lens body upon the opposite entrance and exit ends of which are coated focusing mirrors. At least one of the mirrors, and preferably both, has the configuration of a segment of a circular cylinder. The other mirror may have the configuration of a segment of a circular cylinder or of a sphere. The axes of the mirrors, when both are segments of a cylinder, are perpendicular to each other. Each of the mirrors has a light-transmitting unmirrored pupil aligned along the longitudinal axis of the lens body. When light passes through the entrance pupil, it passes through the lens body, is focused in a given orientation by the exit mirror and reflected back to the entrance mirror where it is again focused in an orientation perpendicular to the first orientation and is reflected toward and out the exit pupil. This focusing mirror lens corrects for the astigmatism of a laser diode and can make highly elliptical beams less elliptical.
A need always exists for additional coupling devices for the efficient coupling of laser diodes to optical fibers. Such a coupling device can also be used to focus any source of radiant energy, especially where the source emits divergent radiation which must be directed to a relatively small aperture.